2nd-generation mobile communication refers to transmission and reception of voice through mobile communication, which includes CMDA, GSM, and the like. GPRS, advancing from the GSM, has been proposed to provide a packet switched data service based on the GSM system.
3rd-generation mobile communication allows for transmission and reception of image and data, as well as voice, and 3GPP (Third Generation Partnership Project) has developed a mobile communication system (IMT-2000) technique and adopts WCDMA as a radio access technology (RAT). The combination of the IMT-2000 technique and the radio access technology (RAT), e.g., WCDMA, is called a UMTS (Universal Mobile Telecommunication System). UTRAN is an acronym of UMTS Terrestrial Radio Access Network.
Meanwhile, the 3rd-generation mobile communication is evolving to 4th-generation mobile communication. Two techniques are issued as the 4th-generation mobile communication technique: one is a long-term evolution network (LTE) technique under standardization by 3GPP and the other is a technique proposed by IEEE. The IEEE 802.16m is called an advanced air interface (AAIF), and both TDD and FDD can be supported by the advanced air interface.
FIG. 1 shows an example of a frame structure according to the IEEE 802.16 technique.
With reference to FIG. 1, a superframe has a length of 20 ms, and each superframe is configured with four radio frames having a length of 5 ms.
The superframe may include a superframe header (SFH). The superframe header includes essential control information which is to be necessarily acquired when a terminal enters a network at an early stage or when terminal performs handover. The superframe header plays a similar role as a broadcast channel (BCH) in the LTE technique. The superframe header (SFH) may be allocated to a first radio frame among a plurality of radio frames constituting the superframe.
Each of the radio frames includes a plurality of subframes. The subframe may be allocated for download and upload transmission. The number of subframes constituting one frame may vary as 5, 6, 7, and 8 depending on the bandwidth of a system or the length of a cyclic prefix (CP). Also, the number of OFDMA symbols constituting a single subframe may vary. First, a type-1 subframe may include six OFDMA symbols, a type-2 subframe may include seven OFDMA symbols, a type-3 subframe may include five OFDMA symbols, and a type-4 subframe may include nine OFDMA symbols. In the two subframe types, some of the symbols are idle symbols.
FIG. 1 shows a case in which when bandwidth are 5 MHz, 10 MHz, or 20 MHz, the length of the CP is ⅛ Tb (Tb: useful OFDMA symbol time).
The frame structure illustrated in FIG. 1 can be applicable to a time division duplexing (TDD) scheme or a frequency division duplexing (FDD) scheme. The TDD scheme refers to a scheme in which an entire frequency band is used as uplink or downlink and an uplink transmission and a downlink transmission are discriminated in a time domain, and the FDD scheme refers to a scheme in which a uplink transmission and a downlink transmission occupy different frequency bands and simultaneously made.
Meanwhile, in a wider broadband mobile communication system, power consumption is a critically significant factor compared with other systems. As one of methods for minimizing power consumption, a sleep mode operation between a terminal and a base station has been proposed.
In the related art sleep mode operation, in a state that the terminal performs communication with a base station in an active mode, when there is not more traffic to be transmitted to or received from the base station, the terminal is changed from the active state to a sleep mode.
Entering the sleep mode state, the terminal receives a message indicating whether or not there is traffic transferred from the base station during a sleep mode listening window (LW), and in this case, when the terminal receives a negative indication indicating that there is no traffic, the terminal determines that there is no data traffic transmitted to downlink, and increases a current sleep mode cycle.
In addition, when the terminal receives a positive indication from the base station during the LW, the terminal determines that there is data traffic transmitted to downlink, so it initiates the current sleep mode period.
FIG. 2 illustrates a general sleep mode operation.
When there is no more data traffic to be transmitted or received in a normal mode, the terminal transmits an SLP-REQ message requesting changing to a sleep mode to the base station (S11), receives an SLP-RSP message including a sleep mode operation parameter such as a sleep cycle, a listening window, and the like, from the base station (S13) to change the state into the sleep mode.
When the terminal first changes the state into the sleep mode, it applies a first sleep cycle SC1 including only a first sleep window SW1 to operate the sleep mode. After the first sleep cycle SC1 is terminated, the terminal applies a second sleep cycle SC2 including a second listening window LW2 and a second sleep window SW2 to operate the sleep mode.
When the terminal receives a TRF-IND message including a negative indication during the second listening window LW2 in the second sleep cycle SC2 (S15), the terminal determines that there is no data traffic transmitted to downlink, and maintains a sleep mode doubled compared to the first sleep cycle SC1.
After the doubled sleep cycle SC2 is terminated, when the terminal receives a TRF-IND message including a positive indication during a third listening window LW2 of a third sleep cycle (S17), the terminal extends (ELW3) the listening window in order to receive generated data traffic, and receives data traffic from the base station (S19), and then enters a sleep interval SW3 to perform a sleep mode operation. In this case, as illustrated, the third sleep mode interval SC3, including the listening window LW3, the extended listening window ELW3, and the sleep window SW3, is reset into the first sleep mode interval SC1.
Meanwhile, a wireless channel has abnormal characteristics such as a path loss, noise, a fading phenomenon due to multipath, an intersymbol interference (ISI), or Doppler effect due to terminal mobility, or the like. Thus, various techniques are being developed in order to overcome the abnormal characteristics of the wireless channel and enhance reliability of radio communication.
An adaptive modulation and coding (AMC) scheme is a technique for enhancing reliability of radio communication. In order to support AMC, a mobile communication system may use a channel quality indicator (CQI). The CQI is information regarding a channel state between a base station and a terminal. The base station determines a modulation and coding scheme (MCS) used for a transmission by using the CQI received from the terminal. When the base station determines that a channel state is good based on the CQI, the base station may increase a modulation order or a coding rate to increase a transfer rate. When the base station determines that a channel state is not good based on the CQI, the base station may reduce the modulation order or the coding rate to lower the transfer rate. When the transfer rate is lowered, a reception error rate can be reduced.
Information regarding channel quality is used as essential information in determining an adaptive modulation and channel coding (AMC) level with respect to the corresponding terminal. Thus, the terminal must transmit the channel quality indicator (CQI) to the system at every frame.
As for a periodical transmission of the CQI, the CQI is transmitted at a period given by a base station or at a predetermined period without a request from the base station. When the CQI is periodically transmitted, CQI information amount, a modulation scheme, a channel coding scheme, and the like, may be previously determined. In this case, overhead of signaling required for the CQI transmission can be reduced.
However, the periodical transmission of the CQI is made when the terminal is in the normal mode.
FIG. 3 shows the problem of the related art.
As noted in FIG. 3, in the related art, the terminal can transmit the CQI while it is operating in the listening window LW, but cannot periodically transmit the CQI when the terminal is operating in the sleep window SW.
In order for the base station to draw a reliable MCS level, the base station must acquire a plurality of CQI from the terminal. However, as illustrated, for example, in order for the base station to transmit data during the second listening window LW2, it needs to acquire a sufficient number of CQIs. However, the base station receives CQIs only during the first listening window LW1 and cannot receive CQIs during the sleep window SW, resulting in that the base station cannot acquire sufficient CQIs.
In addition, the interval between the listening windows LWs is very long. In a situation that channels are changed suddenly, the suddenly changing channel situation cannot be suitably coped with only with the CQI information received only during the listening window. In addition, because the interval between the listening windows is very long, the base station cannot properly know about an average channel situation only with the CQIs received during the listening window.
In addition, as shown, the difference between a time point at which the base station receives a CQI and a time point at which the base station transmits data at the second listing window is too short.